Analysis of fungal genome sequences has revealed the existence of a number of genes that are predicted to be biosynthetic genes for secondary metabolites, but production of a protein encoded by the gene (biosynthetic enzyme for secondary metabolite) has not been identified.
To obtain a protein encoded by a genome sequence, it is ordinarily necessary to first prepare mRNA, synthesize cDNA with reverse transcriptase, and then introduce the cDNA into an expression vector. In general, synthesis of full-length cDNA is quite difficult if a gene has a giant reading frame, so that there may be some reading frames that cannot be covered by cDNA libraries. Also it is difficult to introduce and express such a gene in a host that is different from the source organism (heterologous expression).
Many secondary metabolites have already been used as lead compounds for drugs, and examples of secondary metabolites that have been used in this way include natural polyketides and peptides. These natural products are known to be biosynthesized by polyketide synthases (PKS) and nonribosomal peptide synthetases (NRPS), respectively (Non-Patent Documents 1 to 3).
Regarding the genes found in fungal genome sequences that are predicted to be biosynthetic genes for secondary metabolites, it is anticipated that the secondary metabolites synthesized by the proteins encoded by the genes will be useful. However, because fungi are eukaryotes and their genes contain introns, and the genes are very large, it is difficult to synthesize a full-length cDNA by conventional methods as described above. It has not been possible to synthesize the proteins encoded by genes that are predicted to be biosynthetic genes for secondary metabolites.
Accordingly, there is a need for methods for removing the introns from a fungal giant biosynthetic gene, and expressing a protein encoded by the gene.    Non-Patent Document 1: Leadlay, P. et al., Nature, 1990    Non-Patent Document 2: Katz, L. et al., Science, 1991    Non-Patent Document 3: Samson, S. et al., Nature, 1985    Non-Patent Document 4: Hisao Moriya et al., PLos ONE 2010